1. Field of the Invention
This invention relates to an EGR (exhaust gas recirculation) rate estimation system for an internal combustion engine, and more particularly to a system for estimating a rate of recirculated exhaust gas that enters the combustion chamber in the intake air supplied to the engine.
2. Description of the Prior Art
It is known in automotive engineering to connect the intake system and the exhaust system of an engine for recycling a portion of the exhaust gases to the intake system in order to reduce the formation of nitrogen oxides (NOx) and at the same time, to enhance fuel economy. The passage connecting the exhaust system to the intake system is provided with a valve for regulating the amount or flow rate of recirculated exhaust gas. The valve is hereinafter referred to as the "EGR control valve". In order to control the amount or flow rate of exhaust gas to be recirculated, a command value for the EGR control valve lifting amount is determined based on engine operating parameters at least including engine speed and engine load. In the vacuum-operated EGR control valve, the negative pressure in the intake system is introduced and exerts a negative pressure on the valve to open it in response to the command value, whereas the valve is operated to open via a motor when the valve is of the power-operated type. Here, the term "lifting" or "lift" is used to mean the opening area of the EGR control valve.
As illustrated in FIG. 18, the actual behavior of the EGR control valve lags behind the time that the command value is issued. Namely, there is a response delay between the actual valve lifting and the command value to do so. The lag time is constant in the motor-operated valve, but varies with engine operating conditions in the vacuum-operated valve. Moreover, it takes additional time for the exhaust gas passing through the valve to enter the combustion chamber. Therefore, in order to carry out EGR control properly, it becomes necessary to estimate accurately the amount or flow rate of the recirculated exhaust gas. The amount or flow rate of recirculated exhaust gas is generally estimated as a rate in the mass or volume of intake air or mixture. The rate is referred to as "EGR rate" or "exhaust gas recirculation rate" in the specification, which will be explained in detail later.
Moreover, since the recycled exhaust gas becomes a disturbance in the fuel metering or air/fuel ratio control in an engine, it is proposed in Japanese Laid-open Patent Application Sho 60(1985)-169,641 to determine a fuel injection correction coefficient in response to the EGR rate and to correct the fuel injection amount in a decreasing direction. The prior art system takes into account the fact that the lag time varies with the engine operating conditions in the vacuum-operated EGR control valve, and it delays switching the correction coefficient for a period determined by the engine operating conditions. Another reference, Japanese Laid-open Patent Application Sho 59(1984)-192,838 teaches to change the value of a fuel injection correction coefficient gradually.
Since, however, the behavior of the exhaust gas is more complicated, the assignee proposed earlier in Japanese Laid-open Patent Application Hei 5(1993)-118,239 (filed in the United States and patented under the number of U.S. Pat. No. 5,215,061) to establish a model describing the behavior of the exhaust gas. In this earlier proposed system, the exhaust gas behavior caused by valve lifting is expressed by an equation and the net EGR rate is determined by this equation. Specifically, the earlier system used a concept of two kinds of ratios named "direct ratio" and "carry-off ratio". The first ratio corresponds to a fraction, to the exhaust gases passing through the EGR control valve during the control cycle n, of the exhaust gas that enters the combustion chamber during the cycle n, while the second ratio corresponds to a fraction, to the exhaust gases which passed through the valve by a time m cycles earlier (m.gtoreq.1), but that remained in a space before the combustion chamber, of the exhaust gas that enters the combustion chamber during the cycle n. The earlier system estimates the net EGR rate by the direct ratio and the carry-off ratio.
In addition, the assignee proposed in Japanese Patent Application No. Hei 5(1993)-296,049 (filed in the United States on Oct. 31, 1994 under the number of 08/331746 still pending) another system to estimate the net EGR rate in such a manner that: EQU Net EGR rate=EGR rate at a steady-state.times.(Actual valve lifting amount/Command value for valve lifting amount)
The system configuration is less complicated than that proposed in the publication 5-118,239, since the net EGR rate is estimated from the ratio between the command value and the actual value of valve lifting. (In order to distinguish the EGR rate at a steady-state, the EGR rate is sometimes referred to as the "net" EGR rate.)
However, the amount or flow rate of recirculated exhaust gas depends not only on the amount of valve lifting (the opening area of the valve), but also on engine operating conditions. In other words, the amount or flow rate of recirculated exhaust gas varies with the engine operating conditions even when the amount of valve lifting remains unchanged. The estimation accuracy of the system is therefore not always satisfactory. As a result, the correction coefficient for the fuel metering or air/fuel ratio control calculated on the basis of the estimated net EGR rate is not always proper.
Furthermore, Japanese Laid-open Patent Application No. Hei 4(1992)-311,643 discloses a system for estimating the partial pressures of air and exhaust gas in the intake pipe respectively and based on the estimated values and additionally on the engine speed, to calculate the amount of air entering the combustion chamber. The prior art system needs, however, to determine the amount of exhaust gas recycled in the intake pipe as well as the intake air temperature and the volume of a space (the so-called "chamber") before the combustion chamber and hence requires complicated calculations. It is quite difficult to accurately measure the recycled exhaust gas flow rate due to the delay in the recycled exhaust gas etc. and the calculations are subject to uncertainties.
A first object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which is simple in configuration without requiring complicated calculation, but that is able to estimate the exhaust gas recirculate rate with high accuracy, while reducing calculation uncertainties as much as possible.
As mentioned above, the recirculated exhaust gas will be a disturbance for carrying out the fuel metering or air/fuel ratio control in an engine.
A second object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which enables to estimate the exhaust gas reticulation rate with high accuracy, thereby enhancing the accuracy of the fuel metering or air/fuel ratio control in an engine.
Moreover, when a command value for valve lifting amount is made zero to discontinue the EGR operation, the amount of actual valve lifting does not immediately decrease to zero due to a response delay in the operation of the EGR control valve. The exhaust gas continues to pass through the valve all the while, although the amount or flow rate of exhaust gas passing therethrough is quite small. In addition, when a command value for valve lifting amount becomes zero, there may arise a problem which could make the EGR rate estimation difficult.
A third object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which is able to estimate the exhaust gas recirculation rate correctly, accounting for the delay in the valve operation and without causing any difficulty in the estimation when a command value for valve lifting amount is made zero.
Apart from the above, some engines have a recirculation passage connecting the exhaust system to the intake system at a position relatively farther from the combustion chamber such that a transport delay of the recycled exhaust gas could occur. The transport delay affects the EGR rate estimation.
A fourth object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which can estimate the exhaust gas recirculation rate accurately when the exhaust gas transport delay could occur.
As mentioned repeatedly, the recirculated exhaust gas will be a disturbance for carrying out the fuel metering or air/fuel ratio control and the transport delay could degrade the accuracy of such control.
A fifth object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which can estimate the exhaust gas recirculation rate when the transport delay could occur, thereby enhancing the accuracy of the fuel metering or air/fuel ratio control in an engine.
Aside from the above, the recycled exhaust gas will degrade the ignitability of the mixture in the combustion chamber. Thus, the recycled exhaust gas not only affects the fuel metering or air/fuel ratio control, but also affects the ignition timing control in an engine.
A sixth object of the invention is therefore to provide an EGR rate estimation system for an internal combustion engine which can estimate the exhaust gas recirculation rate accurately, thereby enabling the ignition timing under EGR operation to be determined properly.